Lack of insulin results in reduced seladin-1 expression in primary cultured neurons and in cerebral cortex of STZ-induced diabetic rats.

Several studies demonstrated that Diabetes mellitus (DM) enhances the risk for Alzheimer's disease (AD). Although hyperglycemia and perturbed function of insulin signaling have been proposed to contribute to AD pathogenesis, the molecular mechanisms behind this association is not clear yet. Seladin-1 is an enzyme catalyzing the last step in cholesterol biosynthesis converting desmosterol to cholesterol. The neuroprotective function of seladin-1 has gained interest in AD research recently. Seladin-1 has anti-apoptotic properties and regulates the expression of β-secretase (BACE-1). Here we measured seladin-1 mRNA and protein expressions in rat primary cultured neurons under diabetic conditions and also in the brains of rats with streptozotocine (STZ)-induced diabetes. We show that constant lack of insulin for 5days decreased seladin-1 levels in cultured rat primary neurons. Similarly, a decrease in seladin-1 was found in the brains of rats with STZ-induced diabetes. However, if the lack of insulin and/or high glucose treatment was intermittent, neuronal seladin-1 levels were not affected in vitro. On the other hand, treatment of neurons with metformin resulted in a significant increase in seladin-1. Constant lack of insulin for 5days, as well as high glucose treatment, increased the neuronal expression of BACE-1 in vitro, but not in the in vivo model. Our study defines insulin as a regulator of seladin-1 expression for the first time. The relevance of these findings for the association of DM with AD is discussed.